This invention relates to covert communication systems, i.e., a communication system in which the transmitted waveform exhibits no man-made detection features.
Many military operations involve platforms that wish to communicate without disclosing their presence. Covert communication systems, systems which deny the enemy even the fact that a communication system is in use by friendly units, have been designed using direct sequence pseudo-random noise (DSPN) modulation techniques. Systems employing DSPN modulation techniques produce signals which sound like noise in conventional narrow band receivers. However, if the intercept receiver is situated relatively close to the transmitter, DSPN modulation techniques do not prevent a clean reception of the wideband signal. In this situation an intercept receiver may employ feature detectors to extract tell-tale parameters of the signal, such as spectral shape, identifiable amplitude characteristics or suppressed carrier frequency, which identify it as a man-made signal, defeating the covert communication system.
Another covert communication technique is the use of a transmitted noise reference system. Under the transmitted noise reference system, two channels are broadcast: a reference noise channel and a data channel.
The two transmitted signals sound and look like noise and are correlated at the receiver to extract the data or message signal. In order to achieve covert communication, the reference noise channel must be operated at levels below ambient noise. Because the receiver must work with a reference signal that has been corrupted by external noise, operating at these levels results in degraded performance. Under these conditions, signal processing time increases dramatically.
It is therefore an object of the invention to provide a covert communication system which transmits data and/or voice communications where the transmitted waveform exhibits no man-made detection features.
It is another object of the present invention to provide a covert communication system where the signal may be reduced to much lower levels below the ambient noise levels than in prior systems.
It is also an object of the present invention to defeat all feature detections while minimizing the time it takes for the intended receiver to acquire the signal.